


Gaster's Error

by WelpThisIsMyLifeNow



Series: Art Trades, Commissions, Gifts, and One-Shots [5]
Category: Undertale (Video Game)
Genre: ALL THE FLUFF, Birthday, Fluff, Other, Reader's Gender Unspecified
Language: English
Status: Completed
Published: 2021-02-21
Updated: 2021-02-21
Packaged: 2021-03-18 10:40:55
Rating: Mature
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 4,408
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/29608185
Author URL: https://archiveofourown.org/users/WelpThisIsMyLifeNow/pseuds/WelpThisIsMyLifeNow
Summary: You were Gaster's greatest error.A birthday fic for dear Kitkatchild <3
Relationships: W. D. Gaster/Reader
Series: Art Trades, Commissions, Gifts, and One-Shots [5]
Series URL: https://archiveofourown.org/series/1312277
Comments: 2
Kudos: 44





	Gaster's Error

**Author's Note:**

  * For [Kitkatchild](https://archiveofourown.org/users/Kitkatchild/gifts).



There was something decidedly… unusual going on.

The famous Doctor Gaster was beyond a perfectionist, bordering on the neurotic (and _greatly_ surpassing the annoying, in your humble opinion). Every bit of the project the two of you worked on was meticulously planned, with back ups of back up plans for any possible scenario that were to occur.

So… how did you end up out in a storm with him, desperately trying to save the project you’d been working on for _months_ from a potential flood?

Dr. W.D. Gaster did _not_ make mistakes.

The only error you’d ever see make, in fact, was _you._

The convention had been boring, boring, _boring._ You’d been invited as a guest speaker in your chosen field, giving a bloody lecture on the local _mineral content_ of the soils of Mt. Ebott, of all things. There had been a few interesting presentations you’d caught here and there—but the day had, on the whole, been an utter snooze fest.

So you did the only thing a same person would do under these conditions: go to the bar, and drink away your problems. The allotted two drinks your company had given you were _supposed_ to be for lubricating the social wheels for networking, but… 

You _really_ just wanted to drink away the headaches of the fluorescent lights and all of the people high on their own supply. Although there were a few good eggs in the bunch, the amount of scientists that thought _they_ were the greatest thing since Hawkins just made the whole thing unbearable. 

The bartender scanned the badge the convention had all its attendees wore, ticking off one of the two free drinks. You decided to take it slow; you weren’t a big drinker (and a bit of a lightweight), and (though you enjoyed a slight buzz), you didn’t want to make a fool of yourself. For an indeterminate, booze-sipped stretch of time, you sat at the bar alone, attempting to ignore the idle chatter of various research topics and debates around you.

It was there, fingers idly drumming on an (oddly sticky) bartop, that you met the tall, imposing skeletal monster. 

Although you had been successfully droning out _most_ of the talk around you for quite some time, an _especially_ loud voice cut through the pleasant buzz you had going on. Swiveling on your barstool, you spied the tall monster get his ear talked off ( _he didn’t seem to have any ears, actually, so you might’ve been too late on that one)_ by some overzealous lad. Though the skeleton appeared to be feigning polite interest—even from your distance several feet away—you could tell he was _miserable_ and clearly looking for an exit.

A sucker for a stranger in need, you decided to grant him some mercy. 

Formulating a plan, you decided to inconspicuously get closer to the two—and, like all the other members of the convention (including yourself), he was wearing the badge that displayed his name. Coming up to the two, you looked at the man who was still going on and on and _on_ —and then tapped him on the shoulder.

“Mister-” your eyes briefly flashed to his badge “Drikens, yes?”

“ _Doctor_ Dirkens,” he corrected. Utterly used to this, you didn’t bat an eye at the curt tone. “But yes, I am. How can I help you?”

“Congratulations—you won the raffle! Please go to the convention hall to collect your prize.”

“Raffle?” he asked, looking baffled. “What raffle?”

Tilting your head, your stare gave the message of _what, really, you didn’t know?_ “The raffle put on by the convention? Did you not hear about it during the welcome ceremony?”

“Ah,” he said simply. “Er. Well. That’s great! Excuse me, Dr. Gaster, I suppose I should collect it.”

The monster—Dr. Gaster—nodded. The two of you stood there until the man had ambled his way well out of earshot.

"I can’t say I recall any sort of raffle mentioned,” Dr. Gaster said. His voice was… slightly odd; you _swore_ you heard something akin to the barest echo in the deep notes of his voice. Different, but not unpleasant. You looked to him and smiled. 

“Almost no one pays attention during the welcome ceremonies; they go on for an hour too long.”

Dr. Gaster raised a browbone—he appeared to have cracks going through his skull, and they flexed slightly with the motion. Interesting. “I thought they were only an hour long?”

“Exactly,” you grinned. His mouth morphed into a smirk, sharp eyelight regarding you with amusement. “Just decided to help you out a bit. Enjoy the rest of your night, Dr. Gaster.”

You went to turn off, deciding that was a good deed accomplished; but his voice caught you. 

“A moment, if you will?”

You indeed paused, raising a brow in question. 

“That was quite charitable of you, my dear. Perhaps I could buy you a drink as a thank you?”

The offer slightly jarred you; lest you were mistaken, there was an invitingness behind his tone that hinted at something, perhaps, more than just thankful. After a quick beat of analyzation—yo smiled, and nodded. 

“I think I’d like that, very much.”

So, then, where did Dr. Gaster’s error occur?

Perhaps he would later identify as the error in the invitation itself, or in letting one drink turn into _several_ as the two of you chatted into the night. _Or_ , just maybe, hooking up with you by the vending machines on the third floor. 

The _true_ mistake, as it would turn out, would not be getting your name.

A month or so later—your drunken make out session now evaporated into a hazy (slightly cringey, slightly pleasant) memory, you had decided for a change in course in your life. Working for a company was getting old (and, if you were honest, quite underneath your expertise). Wanting to be increasingly proactive with your skills, you saw the perfect opportunity to put your knowledge to good use: a collaborative research project between monsters and humans. 

Apparently, following the breaking of the barrier, magic had begun to be revitalized throughout the land. While most of the monster's food supply came from crops grown underground, it seemed as if enough magic had permeated the lush soil on the sides of Mt. Ebott that magical crops (such as the ever-popular echoflowers, or the typha “water sausage” crops) might now be cultivated aboveground. 

As a specialist of the land around Mt. Ebott, it was with little surprise that you were selected to join. It would be quite the undertaking; six months on, one month break, then another five months of work—just a year in total. You’d even have to uproot and move closer, but…

This project had real _meaning_. It would be worth it.

Of course, it would be quite the shock when you met the group of scientists you would be working with, _especially_ the lead of the project, who had selected the group members himself:

Dr. W.D. Gaster.

He was (without a doubt, if the nearly voided sockets were any indication) as surprised to see you as you were him. Like true adults, neither of you mentioned a _word_ of it, deciding to ignore the massive, drunken elephant in the room and simply let things stay awkward. That’s not to say neither of you were affected: the mistake clearly haunted him—whenever you cohabitate the same space, it seemed to drive him to all sorts of fumbling and stilted discussions. If it wasn’t for the whispered conversations among your cohorts in regards to his astounding work, you might’ve thought him incompetent.

Well, that _and_ the times he didn’t notice you… his work was _flawless._ He poured every bit of his attention and focus of detail into his work, and watching him at work was mesmerizing. It wasn’t long that you, too, saw what the whispers had been about.

The burden of your presence thankfully eased over an initial mutually frustrating period, likely disappearing as time gave steady evidence of your lack of discussion on the topic. By a few months, he seemed to condition himself out of his erring behavior—that masterful skill winning out over the one night hookup’s spell of lingering awkwardness. 

Over time, you even—dare you say it—slowly arrived on _friendly_ terms. Never once bringing up that night, of course—but (likely inevitably with the long, late hours you worked side by side, usually the last two standing of your group), you two shared bits about your lives. He was indeed brilliant—and while you had little idea what he thought about you, it seemed like he at least thought you were competent, slowly entrusting more of the finer, delicate aspects of the project to you alone. Regardless of how he viewed you, you felt the time you spent was actually rather pleasant. Those late nights were grueling, but… you found you honestly looked forward to it. 

He even outright laughed at a joke of yours once, late at night, his voice echoing out into the otherwise empty lab. You catalogued it as a minor miracle. 

For the six months you were on the project, you hadn’t missed a single day of work. From the theorizing, to the strategizing and filling of permits, to the actual trenchwork of digging in the soil of the mountain, you’d worked _endlessly._ Over the half year, you slowly felt like you’d grown into part of the mountain itself—you more naturally stood at a lean than upright from working on the steep incline, your hands and knees had become calloused and hardened like the rocky soil you dug at, and dirt had slowly accumulated _everywhere_ —on your clothes, in your jeep, in your home. On the worst days, when it felt like even your tastebuds had become dirt (the food the grant supplied was questionable at best), you wondered if dealing in magic’d soil could’ve slowly transformed you into a dirt monster. You regretted not getting into oceanography instead.

But… on the whole, it was worth it. Mudmonster or not, the project was a way to truly help monsterkind—and so your will never faltered. 

By the end of the sixth month—your duties fully explained and prepped to be spread to other colleagues during your absence—you were _beat,_ but knew you had done your absolute best. 

Gaster wasn’t among those who’d shown up to wish you goodbye—but you weren’t particularly surprised, knowing he wasn’t one for sentimentalities. 

When planning for your break, you were allowed to select any month you wanted. In reality, there was only _one_ day that you requested a break, the sole time you felt the weight of the project and the rest of the world could, temporarily, be put aside: 

Your birthday.

So, you picked for the last day of your break to be on your birthday. Call it selfish, or human, or whatever you wanted—but you needed at least that one day to sit, and rest, and pretend that there was a life outside of pipettes and markers and various soil content. Having moved out to center Ebott for the project (perhaps a bit extreme—but not as extreme as Gaster, who’d apparently rented a cabin next to the research site itself), you didn’t have any local friends to make plans with. 

That was just fine with you; having a day of just silent relaxation and enjoying your own peace was more than enough. The majority of your last day of break was spent doing just that: utter bliss took the form of binge watching shows, eating an obscene amount of your favorite trash foods, and blasting music throughout your tiny apartment. For the extra-special finale of your day, you’d planned for an extravagant soak in your bath, accentuated with a glass of cheap wine and the trashiest dimestore novel you could find. It was going to be _perfect-_

But then the phone rang.

You’d answered unthinkingly, figuring it was a family member or a friend likely wishing you happy birthday (or at _worst_ a telemarketer)-

But it was, to your immense surprise, _Gaster_.

After a crackle of static—he always seemed to have phone trouble, for some reason—your heard his unmistakable voice.

“Your assistance is required.”

No how-do-you-do, no “happy birthday,” not even a _hello._ Even if you hadn’t recognized his voice, the lack of pleasantries would have instantly revealed his identity. 

“Oh, I’m doing fine, thank you. But please, tell me how _you’re_ doi-”

“We do not have much time. I need you now.”

“Gaster, can’t you get someone else? It’s my-”

“ _Please._ ”

Well, _that_ was a first. How could you say no, when his voice sounded so desperate?

With bitter, silent laments, you accepted. 

You drove out in the night, rain already battering your windshield _almost_ more than your wipers could fend off. Not for the first time, you were glad you decided to move closer for the work—if you were any further away, you might’ve said it would’ve been impossible to make at all. The night was black, heavy—even the lights that surrounded the plots barely beat out the darkness against the sheets of rain. 

Apparently there was some sort of flood expected—too much, apparently, for the fragile plants to handle. Fortunately, it was only a _few_ of your experimental plots that needed to be dug up—but it would take both time and delicacy. This wasn’t even your area of expertise; why had Gaster called _you?_ Could no one else make it?

You stepped out of the jeep. The rain was harsh and stinging, feeling twice as cold in the unerring darkness. In the muck, there was Gaster—he rarely got his hands so dirty, and seeing him caked with mud was an odd sight. His movements were desperate, clearly racing against a clock with an unknown alarm, so you joined him with only a bare “hey” between you. It was time to work. 

It took well over two hours, but the two of you were able to successfully transport the plants to temporary homes; it took several trips in the large truck that the research team had rented, but eventually, they were all safe. Some got housed in the lab itself—and when you ran out of space there, Gaster said he would bring them into his lodge.

Dropping off the final load to Gaster’s cabin—a tiny, _almost_ picturesque thing (your muddy tracks all over had detracted from it quite a bit). You had never seen it before; it… was about as much as you expected. Organized, tidy (again, until you came in), and looked hardly lived in. He must’ve just slept here. Once complete, you murmured a vague approximation of a goodbye—your bones were weary from the lack of abuse over the past month, and _stars_ , you just needed to go home and get a shower-

“A moment, if you will?”

You instantly froze. It had been so long since that first conversation you’d had at the convention—but your memory, strong and harsh, dragged you back to that moment in an instant.

Likely taking your pause as heeding his interruption of your exit, he spoke again.

“I believe it is far too dangerous to be driving; the flooding is dangerous up here, let alone worse as you head down the mountain. You are an adult and can do as you please, but I would like to offer for you to stay here.”

_Dafuq?_

Poker face failing you, he clearly caught your surprise. 

“Should you accept, I will sleep on the couch. It is the least I can do for interrupting your final day off.”

Hesitation warred within for the briefest of moments. Sure, your friendship had developed into something more stable, but…

...Well, the roads _were_ likely to be rather terrible by now. It was the logical decision.

There was little room for argument, so you nodded. The doctor graciously offered to let you have the first shower; it wasn’t exactly the overindulgent bath you were hoping to take on your birthday, but the gesture was kind enough. He even found you some clothes to wear; apparently, his son (who was also a skeleton, but had a larger frame) tended to visit quite often and left his clothes around the place. Again, it wasn’t the comfy PJ’s you were hoping for, but… it made due.

By the time Gaster had entered the shower, you’d gotten well enough acquainted with the place to feel as comfortable as a strange space allowed; you were currently sat, beleaguered and exhausted, on the plush couch by the fireplace. Despite the shower, you were still chilled down to your core, and huddling by the fire Gaster had lit was slowly seeping into your bones.

It was likely that Gaster hadn’t decorated the place he was renting from; the living room seemed far too cozy for such a regimented soul. An open floor plan from the living room to the kitchen allowed the warmth to linger throughout the space, allowing the heat to not overwhelm despite the brightness of the fire. There were all number of nicknacks and scenic landscape paintings, all homey, sentimental touches. Between the soft crackle of the fire and the steady, almost harmonic beat of rain against the windows, there was an of coziness that was wrapping around you and… pulling you… into...

A hollow, metallic clanging woke you up, startling you out of your stupor. A soft swear sounded out some distance behind you; turning around (mind hazily noting that you had a throw blanket on you—when did that happen?), you saw Gaster rifling through some pots in the kitchen. A scent of acidic tomato was in the air, mixing pleasantly with the hints of smoke from the fireplace. His stare checked in your direction-

“My apologies; I was hoping to let you rest a bit longer.”

Sliding off the blanket—you’d gotten a tad overheated during your nap—you shook your head. “No, that’s quite fine! I appreciate you letting me rest while you cooked. You must be even more tired than I am.”

“Not a problem, I assure you. It was quite charitable of you to come out in the first place on your day off.”

Once again, a flashback of that first night struck you.

_Is he… doing that on purpose?_

Likely a coincidence, you brushed it off for the time being (correlation, not causation and all that). In an attempt to be a proper guest, you spent a few minutes aiding Gaster in what little was left to do with the cooking before sitting down for a meal. 

It was a simple spaghetti, although surprisingly good despite the simplicity of the dish. In a way, this didn’t surprise you either; the dish was functional, likely a repeat meal for Gaster. 

What _did_ surprise you, however, was the spirited conversation you had over dinner. Despite the mutual exhaustion, Gaster seemed to transform back into the monster you’d met that first evening, his company thoroughly enjoyable. It was _almost_ to the point where you wondered if this was the same Gaster at all. 

“Something on your mind?”

Startled, you realized you’d been swirling the same bit of spaghetti onto your fork for the last half minute. You grinned sheepishly.

“I guess… I’m a bit stupefied by just how different you are between here and work.”

He was silent for a long moment, those eyes regarding you thoughtfully. His mouth tugged into a slight frown, though his eyes looked more sincere than upset. 

“Ah. I imagine it is quite the stark difference. This project… is one that cannot fail. When I am at work, I’m afraid I must put my all into it. My… determination, you could say, overrides all else.”

Softening, you smiled with warmth and understanding alike. “And here I thought you just disliked my company.”

That spark within his eyes brightened as he smiled in return. “Quite the opposite, my dear.”

Though you doubted your pleased flush went unnoticed, Gaster at least spared you any mention of it. 

Dinner soon concluded afterwards. Gaster attempted to insist upon doing the cleanup, but—stubborn as ever—you refused to be a poor guest. The two of you compromised, taking up real estate side by side at the kitchen sink. You washed, he dried. 

An amicable, somewhat sleepy silence fell over the two of you. Thoughts lazily drifting through your head, one eventually slipped out without much consideration.

“Hm… You know, it’s rather odd of you not to have caught this disaster in advance.” Your words then struck you, and you cringed apologetically as you handed him a plate. “I don’t mean that as a criticism; I’m sure there’s a good reason. Just… It’s so rare for you to be caught unprepared, you know?”

He wasn’t facing you, though you caught the frown that momentarily caught his mouth, before moving into something more neutral. He was silent for a long, pressured beat. 

_Great going. Why not insult him a little more?_

“I’m sorry-”

“Do you know, my dear, how long behavioral scientists say it takes to break a habit?” he interrupted. You were slightly jarred by the topic change.

“Uh-”

“Your behavioral scientists have had a myriad of answers,” he answered casually, his gaze still cast forward as he continued the task of drying off the dish. “At first it was twelve days, then fourteen, then eighteen, then twenty-one—and then all the way up to sixty-six days.”

Brow raised, you continued your task of cleaning the largest of pots—you had _no_ idea where he was going with this, but figured you would allow him the space to make his point. If he had something to say, he could simply say it.

After a beat of silence, he did indeed continue.

“Upward estimates are five hundred days, depending on the person and the habit. For myself—after much testing with things like cigarettes and alcohol, I’ve found that my habits are generally extinguished around the one week margin.”

The mention of alcohol had your interest piqued—you realized you’d never (in all the six months of team dinners and more social settings) seen him drink after that night. Had he quit? 

“Just one week, huh? Congratulations,” you offered, only a _minor_ amount of sass included. “I suppose I’d expect no less from the great Dr. Gaster.”

A side glance caught the edge of his mouth quirking into a smile—but it was extinguished almost immediately. “Yes, that is what I thought as well.”

The statement was offered dryly, matter-of-factly—unlike most other scientists in his standing, there seemed to be only _minor_ ego attached to his brilliance. You supposed that’s why you got along rather well.

“Are you aware you have been gone for four weeks?”

Handing him the pot, he finally looked your way—and you blinked blankly at him. When he offered no further clarification, you gave him a lame look. “Believe it or not, even us non-quantum scientists _can_ count to four.”

He finally smiled—a huff of a laugh coming out as he eyed you, his eyelight flitting back and forth as it moved over your face evaluatively as he set the pot down. “Yes, I’m well aware _you_ can.” 

There was a teasing tone behind his words—but before you had the chance to get offended on behalf of your field, Gaster took a step closer to you, deftly plying the sponge out of your hand.

“ _You_ , out of all, have been proven to be far beyond expectation.”

You felt your heart thud in your chest at the compliment—but refused to back away with his intrusion, ignoring the heat rising in your chest as you stared back at him unflinchingly. “Thought so low of me, doctor?”

He laughed again—low, surprised—before giving a bare shake of his head. His hip leaned against the counter—edging further into your space without stepping forward. It became increasingly difficult not to falter under that analyzing stare. 

“No,” he said simply. “Not at all.”

Silence again—this one tense, but you had no idea what to say. Blessedly, he broke it for you.

“Do you know why I called you tonight?”

“I assumed because no one else picked up?”

“Because you’re the only one that I trusted to do it correctly,” he supplied. “This month that you’ve been gone has been an utter disaster; no one comes close to your competency, skill, and hard work.”

The utter and blunt sincerity of the statement startled you. “Er, well… Thank you.”

“And I’ll admit,” he continued, “on a personal note… The lab has been rather dull without your presence.” 

Heart picking up beat, you watched as he pulled out a small, rectangular box from his pocket, presenting it to you. “I intended to give this to you tomorrow, but… since we happened to cross paths on the day of…”

“Gaster, jeeze…” You accepted the gift, opening the lid of the case. It was a delicate looking bracelet, a small intricately detailed echoflower at the center—the same ones you were endeavoring to grow together.

Your throat suddenly felt thick, and you swallowed hard. “I… don’t know what to say. Thank you; it’s absolutely lovely.”

“May I?” he asked, gesturing towards it. Nodding, he carefully took it from you, encircling it around your wrist as he clasped it on.

“I must apologize for my behavior these past few months. I never intended to make you think that I did not appreciate your presence—it has been rather the opposite, in fact.”

Flushing deeper, you shivered as the feeling of smooth bone brushed against your wrist, his voice soft and sincere. Despite the sudden shyness in your core, you looked up from the bracelet to meet his stare. “I do too, Gaster.” 

His smile this time was unlike any you’d seen before—not smirking or amusement, but instead, actual tender joy.

“I was wondering, if it wouldn’t be too forward… could I kiss you, my dear?”

“That depends…” you said, unable to fight the urge to tease him. “Do you plan on pretending I don’t exist for another five months, just _happening_ to drop a beaker every time I enter a room?”

He chuckled, his hand reaching out to cup your face. 

“I never intend to make that error again.”

  
  


**Author's Note:**

> A sweet fic for a sweet friend! I hope you have the very happiest of birthdays <3 <3 <3 
> 
> I hope you like it! My apologies that it may be a littler under-edited. It was a lot of fun to write though; I've never written Gaster before! :D 
> 
> Thanks for reading all! :3


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